Electrical machines are known comprising a stator and a rotor integral with a shaft which ensures the movement of a spiral compressor also known as a “scroll compressor”. A system of this type comprises two interposed spirals, such as blades, in order to pump and compress the coolant fluid. In general, one of the spirals is fixed, whereas the other is displaced eccentrically without rotating, such as to pump, then trap, and finally compress pockets of fluid between the spirals. A system of this type is described for example in document EP1 865 200.
The rotor made of laminated plate comprises a central core, and arms which extend radially relative to the core. These are each comprise two rims which extend circumferentially on both sides of the arms. Permanent magnets are positioned inside receptacles, which are each delimited by two faces opposite one another of two adjacent arms, an outer face of the core of the rotor, and the rims of the arms.
When the tolerances used in the production of the rotor are high in order to reduce the production costs, it is possible for the magnets to be badly placed inside the receptacles of the rotor. This can give rise to various problems:                a. Mechanical imbalance if the magnets are not all in the same radial position.        b. Greater stresses on the parts of the magnets which are in contact with the rotor plate, in particular under the effect of high-speed centrifugal forces. Under the effect of these forces, and as a result of an irregularity of the contact surface, a very high level of stress can be generated in the magnet, causing it to break or split.        c. Small fragments of magnet can be ejected from the rotor, and damage the stator under the effect of the centrifugal forces.        
In addition, one of the problems of these machines is the retention of the magnets during centrifugation.